1. Field of the Invention
The present invention relates generally to an FM-CW multibeam radar apparatus for use as a vehicle-mounted millimetric wave radar apparatus and the like. More particularly, it relates to a multibeam radar apparatus involving a combined use of a monostatic scheme in which a beam is transmitted and received by one and the same antenna, and a bistatic scheme in which a beam is transmitted and received by different antennas, wherein the bistatic scheme is used in detecting at least a short-distance-away target to prevent decrease in accuracy of short-distance-away target detection due to diffraction of a transmitted signal from an antenna-shared circuit such as a circulator to thereby render the radar apparatus capable of detecting both short- and long-distance-away targets with high accuracy.
2. Description of the Related Art
In U.S. Pat. Nos. 4,349,823, 5,181,037, 5,229,774 and 5,369,409, the present Assignee has proposed a multibeam radar apparatus in which a plurality of beams with different directivity characteristics are arranged to be radiated in a time divided fashion for detecting not only the distance to a target but also the bearing of the latter.
Reference is made to FIG. 7 in which a conventional FM-CW multibeam radar apparatus is shown in a block diagram. The radar apparatus 100 comprises: a plurality of (e.g., four) beam transmitter-receivers 101A-101D; a transmitted signal switch 104 for switching the destination of a transmitted signal 103a generated by an FM signal generator 103 in accordance with transmitter-receiver channel designating information 102a supplied from a switching control 102; a received signal selector 106 for selecting a beat signal supplied to a distance/bearing detector 105 in accordance with the transmitter-receiver channel designation information 102a; and a sweep control 107 for supplying a frequency designation voltage signal 107a to a voltage-controlled FM signal generator 103 in accordance with a sweep instruction 102b supplied from the switching control 102 and for performing frequency modulation (FM) to the transmitted signal 103a.
Each beam transmitter-receiver 101A-101D comprises: a power distributor 111; a circulator 112, transmitter-receiver shared antennas 113A-113D; and a mixer (mixing circuit) 114. The circulator 112, designed to separate transmitted and received signals, supplies a signal 111a transmitted from the power distributor 111 to an antenna 113 and supplies a signal received by an antenna 113 to the mixer 114. Use of the circulator 112 allows both transmission and receiving to be performed by a single antenna. The mixer 114 mixes a transmitted signal 111b from the power distributor 111 with a received signal 112a supplied via the circulator 112 to output a beat signal BS (BSA-BSD).
FIG. 8 illustrates beam directional characteristics of individual beam transmitter-receivers. Four antennas 113A-113D having substantially identical pencil beams are arranged such that the beams of adjacent two antennas partially overlap with each other. Consequently, the distance to and bearing of the target can be sensed by selectively switching the beam transmitter-receivers 101A-101D for performing transmission/receiving and measuring the frequency and level of a beat signal BS for each pointed direction.
For effecting transmission/receiving with one and the same antenna, a transmitted signal and a received signal need to be separated by the circulator 112. However, since the circulator 112 has limited separation performance, part of the transmitted signal 111a leaks out into the received signal 112a. As a result, a noise component is generated in a low frequency region of the beat signal BS output from the mixer 114.
FIG. 9 is a graph showing the frequency spectrum of a beat signal.
Due to the limited separation performance of the circulator 112, a noise having 1/f (f: frequency) characteristics is generated, whereby the noise level of a particular target becomes larger than the original level of an associated beat signal. Under these circumstances, since the frequency of a beat signal is proportional to the distance to a target, difficulty is experienced in short-distance-away target detection involving beat signals with small frequency values.